1. Field of the Invention:
The present invention relates to a base substance for a magnetic head which writes or reads on a magnetic recording medium, and more particularly to an improvement of the material forming the base substance for a magnetic head suitable for high density magnetic recording.
2. Description of the Prior Art:
The demand for higher density in magnetic recording has grown in recent years with the increase of information available to be magnetically stored. In order to magnetically store more information, improvements of various components, such as magnetic recording heads and novel magnetic recording systems are actively underway.
With regard to the magnetic head, a general purpose ring type magnetic recording head made of a ferritic core was considered limited in its capacity for high density magnetic recording. Thin film magnetic recording heads such as disclosed in Jap. Pat. Appln. Laid-open No. Sho 55-84018 have been developed to replace the conventional heads.
The core material used to make these thin film magnetic heads is made of permalloy which has marked magnetic permeability at high frequency and saturated magnetic flux density as compared with a ferritic core. The magnetic field of the head which is responsible for recording switches very rapidly and is capable of recording with high resolution. Because of smaller inductance, therefore, it can be used over a wider frequency range. Conveniently the principal components of the thin film-type magnetic heads are formed by a vacuum evaporating process or a sputtering process. Such heads are easier to produce and require less mechanical workings and therefore offer an excellent magnetic head.
The combination of a thin film magnetic recording head having excellent characteristics as mentioned above with a magnetic recording medium of metallic thin film or of film coated with .gamma.-type diiron trioxide adherent to cobalt will allow high density magnetic recording.
FIGS. 1 through 3 show the structure of a conventional thin film magnetic head. In FIGS. 1 through 3, reference numeral 1 denotes a non-magnetic base substance in a plate form which is made of an alumina ceramic 1 to 5 mm thick having a sufficiently smooth surface for evaporating a film of permalloy 2 thereon. The permalloy film 2 is the bottom magnetic pole made of a magnetic soft metal film is formed on the surface of the non-magnetic base 1 by evaporating or sputtering. An alumina film 3 which is a non-magnetic insulation film is formed over the permalloy film 2 by sputtering to act as a gap spacer.
The core is formed as follows: a thick photo-resist film 4 is superposed on the alumina film 3 and consecutively superposed with a thin film coil 5 of a one layer spiral having eight turns. A photo-resist film 6 is formed over the thin film coil 5, and a permalloy film 7 as the upper magnetic pole is further evaporated on the photo-resist film 6.
As shown in FIG. 3, an alumina film 8 is formed by sputtering the permalloy film 7 as the upper magnetic pole to protect the thin film coil 5. A non-magnetic protective plate 10 is bonded to the upper surface of the alumina film 8 by means of a glass layer 9 which is adhesive at a relatively low temperature. The non-magnetic protective plate 10 is less than 100 .mu.m in thickness, protects entire thickness from the permalloy film 2 as the bottom magnetic pole to the alumina film 8 and is made mainly of silica (SiO.sub.2). The non-magnetic protective plate 10 and the upper surface of the non-magnetic base 1 (the surface is indicated as B in FIG. 3) come in contact with the magnetic recording medium. Since the non-magnetic protective plate 10 is relatively thinner than the non-magnetic base 1, it is the non-magnetic base 1 which primarily contacts the magnetic recording medium. Reference number 11 in FIG. 1 denotes the magnetic recording medium which comprises a magnetic layer 11a and a non-magnetic layer 11b. In FIG. 2, reference numerals 12 and 13 denote electric signal terminals.
Since the upper surface of the conventional non-magnetic base for a thin film magnetic head is formed by evaporation or sputtering, alumina ceramics are often used for the desirable property of providing a smoother surface for the base. However, alumina ceramics are so hard that the surface of the recording medium which slides over the non-magnetic base is likely to be injured. Particularly when the thin film magnetic head is used as the recording medium for a metallic thin film magnetic disc or a magnetic tape, the metallic film surface of the recording medium is less coarse and thus the actual area in contact with the thin film magnetic head increases. This deteriorates the slidability between the magnetic head and the recording medium and causes damage to the recording medium.
With regard to the magnetic recording system, it has been logically proven that the conventional longitudinal magnetic recording system is also limited in its capacity for high density recording (Iwasaki, Institute of Electronics and Communication Engineering of Japan, Research Information on Magnetic Recording MR72-7, 1972, 6; and S. Iwasaki & K. Takamura, IEEE Tras. on Magn. MAG-11, No. 5, 1173-1175, 1980). A perpendicular magnetic recording system has since been proposed as an alternative system for increasing the recording density.
Referring to FIG. 4, it is seen that the longitudinal magnetic recording system has the axis of easy magnetization in the direction the recording medium 11 advances denoted as arrows with broken lines. As the recording density is increased, circular modes of magnetization 20, as indicated by the arrows in FIG. 5, will be generated because of the demagnetizing field. When these circular modes of magnetization approach each other, the direction of their rotation becomes undiscernible, making magnetic recording and reproducing impossible.
On the other hand, the perpendicular magnetic recording system as shown in FIG. 6 gives rise to parallel magnetism in which the direction alternates at every half-wave length of the signal. The adjacent residual magnetization blocks attract each other so that the system stays stable, is free from the demagnetizing field which is the source of problem in the longitudinal magnetic recording, and thus causes the magnetism to switch sharply. Since this switching of the magnetism is also retained even when the recording density is increased, the perpendicular magnetic recording system utilizes the attraction force of the magnet and is therefore very rational and advantageous in increasing the density of the magnetic recording.
FIG. 7 shows the fundamental structure of a magnetic head for use in perpendicular magnetic recording system. The conventional magnetic head comprises a main pole head 30 vertically in contact with the surface of the recording medium 11 and an auxiliary pole head 40 not in contact with the recording medium 11. The main pole head 30 is less than 1 .mu.m in thickness and is a thin film made of magnetic soft metal usually surrounded by a support member 50 for protection. Reference numeral 60 denotes electric signal terminals.
Attempts have been made to employ thermosetting resins, such as epoxy resin, or thermoplastic resins, such as methylmethacrylate resin, for the support member 50 which protects this thin film of magnetic soft metal. However, these materials are defective in that the life of the support member becomes shorter because these resins lack the hardness to withstand the abrasion when the Co-Cr sputtering film, which is generally used as the recording medium in a perpendicular magnetic recording system, moves in the direction of the arrow in FIG. 7 in contact with the resinous surface.
The use of very hard alumina ceramics as the support member has been proposed to obviate such problems, but in this case, both the recording medium and the support member are so hard that use of lubricant becomes indispensable to insure smooth sliding between the two surfaces. Fluorocarbon compounds, such as Krytox manufactured by DuPont, USA, and amide compounds can be used as the lubricant to be applied on the support member at the time of sliding. Slidability of the Co-Cr sputtering film with alumina ceramics can be improved by such lubricants. However, it was difficult to retain its action for a long time as the lubricant easily volatilizes.
There is also an increasing demand for higher density in the longitudinal magnetic recording system that is widely used in audio tape recorders and video tape recorders in conjunction with the trend toward higher performance and extremely compact devices.
It is necessary to use a magnetic recording medium of high coercive force for realizing higher recording density, and in turn development of a magnetic head having greater magnetic intensity at the time of recording is desired. In other words, a magnetic head which is compatible in performance with the magnetic recording medium of higher coercive force must be capable of generating a strong leakage magnetic field at the front gap. The core material for a general purpose video magnetic head is at present a magnetic material. Mn-Zn ferritic material is typically used as the head material for its balanced properties and workability. However, the head chip of a magnetic head made of oxidized magnetic material would likely be saturated if the current for recording is increased to magnetize the recording medium of high coercive force such as metal tape, and this would cause the recording magnetic field to expand too much to sufficiently magnetize the tape, rendering the head incompatible with metal tape.
Generally, the saturated magnetic flux density (Bs) of magnetic material for the magnetic head is required to be greater than the coercive force (Hc) of the magnetic recording medium a factor of at least 6. For example, a magnetic head made of Mn-Zn ferrite whose Bs is approximately 5000 G (Gauss) cannot sufficiently record on a magnetic recording medium with coercive force of about 1000 to 2000 Oe (Oersted).
A magnetic head made of a magnetic metal material, such as sendust alloy or an amorphous alloy of the Co-Nb-B group, which is about twice as great in Bs as the oxidized magnetic material, such as Mn-Zn ferrite, will be adequate to record on a recording medium with Hc of up to about 15000 Oe. Development of magnetic heads using a magnetic metal material of this type has been active in recent years, but magnetic metal materials are defective in that: (1) theei resistivity is very small; (2) magnetic permeability at the higher frequency band is low; and (3) they are inferior in wear resistance. Among these defects, (1) and (2) can probably be dealt with by forming the magnetic metal material into a thin strip, which in turn provides a magnetic head with a narrow track width suitable for higher density recording. However, the problem of inferior wear resistance as mentioned in (3), above, still remains unsolved even if the metal is made into a thin strip.
Attempts have been made to overcome the problem by supporting the thin strip of metal with a material having excellent wear resistance. Alumina ceramics and very hard glass have been proposed as materials for such a support member which is excellent in wear resistance, very hard, have a smooth surface and are readily available. However, the resulting magnetic head incorporating a support member of this type is too hard and lacks the required slidability with the magnetic recording medium and thus causes damage to the same.
In order to overcome the above-mentioned defect, it has been suggested to apply lubricant over the surface of the magnetic recording medium or to coat the medium with a protective film. The use of lubricant is disadvantageous in that it is difficult to retain the lubricating action for a long time because the lubricant volatilizes and requires periodic re-application. The protective film is defective because it involves an extra process for applying such a film and the protective film adds an extra gap between the magnetic head and the recording medium thereby making higher density recording difficult.